1. Field of the Invention
This invention pertains to solid electrolyte chip capacitors particularly useful in thick and thin film hybrid circuitry and surface mounted devices technology. Such components must have a small size, high volumetric efficiency, compatibility with automatic component placement equipment for surface mounting and the high temperatures encountered with reflow soldering. The invention is directed in particular to maximizing the volumetric efficiency while minimizing the manufacturing cost of such a device.
2. Description of the Prior Art
Solid electrolyte chip capacitors in the prior art have utilized solid tantalum capacitor bodies with various means for forming conductive terminations to provide polarized devices. Four principal types of construction of chip solid electrolyte capacitors are known in the prior art. The T-bar construction is least expensive to manufacture and offers reasonable volumetric efficiency. It sacrifices efficiency in surface area on a PC board, is fragile and is not easily adaptable to automatic handling in placement. An alternative metal case T-bar suffers from decreased volumetric efficiency, decreased surface area efficiency and high manufacturing costs. Molded solid electrolyte capacitors offer reasonable efficiency in surface area but have the poorest volumetric efficiency. Molded capacitors tend to be dimensionally uniform and mechanically protected but they are sensitive to silver scavenging and offer poor packaging efficiency. However, they are well adapted to mechanized manufacture and to automatic placement equipment.
Conformal coated solid electrolyte chip capacitors offer the best volumetric efficiency, the best efficiency in use of surface area on a substrate or PC board and are mechanically protected. They are also insensitive to silver scavenging and some, such as that disclosed in U.S. Pat. No. 3,855,505, are very reliable through thermal stressing. Furthermore, the conformally-coated capacitors with metal end cap terminations (U.S. Pat. No. 3,855,505) are the best adaptable to automatic placement and are adaptable to the various surface mounting methods presently known. The major drawback of the conformal-coated capacitor is its high manufacturing cost. The provision of metal end cap terminations for these capacitors is labor-intensive. In the case of plated end caps, ESR may be a problem at the positive end. The required precision of the plated pads and the uniformity of length of the units are not dimensionally stable. These units are difficult to use with automatic surface mounting equipment.
The present invention has for its object the cost-effective manufacture of a solid electrolyte chip capacitor for automatic surface mounting which incorporates advantages found in various types of conformally-coated chip capacitors in the prior art while eliminating the various disadvantages thereof and achieving a reduction in the cost of manufacture of such capacitors.